I'm about to start experimenting with a generic "fits anything" basing pattern.
A few small patches of grey gravel, a few patches of sand, and a few patchy sections of grass, plus a rock or two.
That way it could be a small tuft of grass poking out from rocks in the desert, an overgrown broken area in an urban setting, or an area where the grass has worn thin if the model is in grassland.
If you took your urban base. gave it a couple of patches of sand from the desert base, and added a couple of tiny patches of green flock or static grass, I reckon you'd get something that will look good anywhere. You could try it out on an empty base to see how it looks on various tables.